THE FORMATION OF WOOD IN PLANTS. 429 
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mittent compressions caused by oscillations urge the sap along the vessels and ducts. 
And it is also experimentally proved that the same intermittent compressions produce 
exudation of sap from vessels and ducts into the surrounding tissue. 
That the processes here described, acting through all past time, have sufficed of them- 
selves to develope the supporting and distributing structures of plants, is not alleged. 
What share the natural selection of variations distinguished as spontaneous has had in 
establishing them, is a question which remains to be discussed. Whether acting alone 
natural selection would have sufficed to evolve these vascular and resisting tissues, I do 
not profess to say. That it has been a cooperating cause, I take to be self evident: it 
must all along have furthered the action of any other cause, by preserving the indivi- 
duals on which such other cause had acted most favourably. Seeing, however, the con- 
clusive proof which we have that another cause has been in action—certainly on indi- 
viduals, and, in all probability, by inheritance on races—we may most philosophically 
ascribe the genesis of these internal structures to this cause, and regard natural selection 
as having here played the part of an accelerator. 
EXPLANATION OF PLATE LIV. 
Fig. 1. Absorbent organ from the leaf of Euphorbia nerüfolia. The cluster of fibrous cells forming one 
of the terminations of the vascular system is here imbedded in a solid parenchyma. 
Fig. 2. A structure of analogous kind from the leaf of Ficus elastica. Here the expanded terminations 
of the vessels are imbedded in the network parenchyma, the cells of which unite to form enve- 
lopes for them. 
Fig. 3. Shows on a larger scale one of these absorbents from the leaf of Panax Lessonii. In this figure 
is clearly seen the way in which the cells of the network parenchyma unite into a closely-fitting 
case for the spiral cells. 
Fig. 4, Represents a much more massive absorbent from the same leaf, the surrounding tissues being 
omitted. 
Fig. 5, Similarly represents, without its sheath, an absorbent from the leaf of Clusia flava. 
Fig. 6. End view of an absorbent organ from the root of a Turnip. It is taken from the outermost layer 
of vessels. Its funnel-shaped interior is drawn as it presents itself when 
outside of this layer, its narrow end being directed towards the centre of the Turnip. 
Fig.7. A longitudinal section through the axis of another such organ, showing its annuli of reticulated 
‘ cells when cut through. The cellular tissue which fills the interior is supposed to be removed. 
Fig. 8. A less-developed absorbent, showing its approximate connexion with a duct. In their simplest 
forms, these structures consist of only two fenestrated cells, with their ends bent round so as to 
meet. Such types occur in the central mass of the Turnip, where the vascular system is rela- 
tively imperfect. Besides the comparatively regular forms of these absorbents, there are forms 
composed of amorphous masses of fenestrated cells, It should be added that both the regular 
and irregular kinds are very variable in their numbers: in some turnips they are abundant, and 
in others scarcely to be found. Possibly their presence depends on the age of the Turnip. 
VOL, xxy, 3 0 
looked at from the 
